ASTM E1570-00
(Practice)Standard Practice for Computed Tomographic (CT) Examination
Standard Practice for Computed Tomographic (CT) Examination
SCOPE
1.1 Computed tomography (CT) may be used for new applications, or, in place of film radiography, provided that the capability to disclose physical features or indications that form the accept/reject criteria is fully documented and available for review.
1.2 The CT systems addressed in this practice utilize a set of transmission measurements made along a set of paths projected through the test object from many different directions. Each of the transmission measurements within these views is digitized and stored in a computer, where they are subsequently conditioned (for example, normalized and corrected) and reconstructed by one of a variety of techniques. An in-depth treatment of CT principles is given in Guide E1441.
1.3 Computed tomography (CT), as with conventional radiography and radioscopic examination, is broadly applicable to any material or test object through which a beam of penetrating radiation may be passed and detected, including metals, plastics, ceramics, metallic/nonmetallic composite material, and assemblies. The principal advantage of CT is that it provides densitometric (that is, radiological density and geometry) images of thin cross sections through an object. Because of the absence of structural superposition, images are much easier to interpret than conventional radiological images. The new user can quickly learn to read CT data because images correspond more closely to the way the human mind visualizes 3D structures than conventional projection radiology. Further, because CT images are digital, the images may be enhanced, analyzed, compressed, archived, input as data into performance calculations, compared with digital data from nondestructive evaluation (NDE) modalities, or transmitted to other locations for remote viewing, or a combination thereof. While many of the details are generic in nature, this practice implicitly assumes the use of penetrating radiation, specifically x ray and [gamma] ray.
1.4 This practice provides procedural information for performing CT examinations. The techniques and the applications associated with CT examination are diverse. This practice is not intended to be limiting or restrictive, but rather to address the general use of CT technology and thereby facilitate its use.
1.5 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use. For specific safety statements, see Section 8, NBS Handbook 114, and Federal Standards 21 CFR 1020.40 and 29 CFR 1910.96.
General Information
Relations
Standards Content (Sample)
NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
Contact ASTM International (www.astm.org) for the latest information
Designation:E1570–00
Standard Practice for
Computed Tomographic (CT) Examination
This standard is issued under the fixed designation E 1570; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
1. Scope tions. Using Non-Medical X-Ray and Sealed Gamma-Ray
Sources, Energies Up to 10 MeV
1.1 This practice is for computed tomography (CT), which
2.3 Federal Standards:
may be used to nondestructively disclose physical features or
21 CFR 1020.40 Safety Requirements of Cabinet X Ray
anomalies within an object under examination by providing
Systems
radiological density and geometric measurements. This prac-
29 CFR 1910.96 Ionizing Radiation
tice implicitly assumes the use of penetrating radiation, spe-
2.4 ASNT Documents:
cifically x-ray and g-ray.
SNT-TC-1A Recommended Practice for Personnel Qualifi-
1.2 CT systems utilize a set of transmission measurements
cation and Certification in Nondestructive Testing
made along paths through the examination object from many
ANSI/ASNT-CP-189 Qualification and Certification of
different directions. Each of the transmission measurements is
Nondestructive Testing Personnel
digitizedandstoredinacomputer,wheretheyaresubsequently
2.5 Military Standard:
reconstructed by one of a variety of techniques.Atreatment of
MIL-STD-410 Nondestructive Testing Personnel Qualifica-
CT principles is given in Guide E 1441.
tion and Certification
1.3 CT is broadly applicable to any material or examination
2.6 AIA Standard:
object through which a beam of penetrating radiation passes.
NAS-410 Certification and Qualification of Nondestructive
The principal advantage of CT is that it provides densitometric
Testing Personnel
(that is, radiological density and geometry) images of thin
cross sections through an object without the structural super-
3. Terminology
position in projection radiography.
3.1 Definitions—For definitions of terms used in this guide,
1.4 This practice describes procedures for performing CT
refer to Terminology E 1316 and Annex A1 in Guide E 1441.
examinations. This practice is to address the general use of CT
technology and thereby facilitate its use.
4. Summary of Practice
1.5 This standard does not purport to address all of the
4.1 Requirements in this practice are intended to control the
safety concerns, if any, associated with its use. It is the
reliability and quality of the CT images.
responsibility of the user of this standard to establish appro-
4.2 CTsystems are made up of a number of subsystems; the
priate safety and health practices and determine the applica-
function served by each subsystem is common in almost all CT
bility of regulatory limitations prior to use. For specific safety
scanners. Section 7 describes the following subsystems:
statements, see Section 8, NBS Handbook 114, and Federal
4.2.1 Source of penetrating radiation,
Standards 21 CFR 1020.40 and 29 CFR 1910.96.
4.2.2 Radiation detector or an array of detectors,
4.2.3 Mechanical scanning assembly, and
2. Referenced Documents
4.2.4 Computer system including:
2.1 ASTM Standards:
2 4.2.4.1 Image reconstruction software/hardware,
E 1316 Terminology for Nondestructive Examinations
2 4.2.4.2 Image display/analysis system,
E 1441 Guide for Computed Tomography (CT) Imaging
4.2.4.3 Data storage system, and
E 1695 TestMethodforMeasurementofComputedTomog-
4.2.4.4 Operator interface.
raphy (CT) System Performance
2.2 NIST Standard:
NBS Handbook 114 General Safety Standard for Installa-
Available from National Institute of Standards and Technology (NIST),
Gaithersburg, MD 20899.
1 4
This practice is under the jurisdiction of ASTM Committee E-7 on Nonde- AvailablefromStandardizationDocumentsOrderDesk,Bldg.4SectionD,700
structive Testing and is the direct responsibility of Subcommittee E07.01 on Robbins Ave., Philadelphia, PA 19111-5094, Attn: NPODS.
Radiology (X and Gamma) Method. Available from American Society for Nondestructive Testing, 1711 Arlingate
Current edition approved May 10, 2000. Published July 2000. Originally Plaza, P.O. Box 28518, Columbus, OH 43228-0518.
published as E 1570 – 93. Last previous edition E 1570 – 95a. Available from the Aerospace Industries Association of America, Inc., 1250
Annual Book of ASTM Standards, Vol 03.03. Eye Street, N.W., Washington, DC 20005.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
E1570–00
4.3 Section 8 describes and defines the procedures for causes artifacts such as beam hardening (the anomalous
establishingandmaintainingqualitycontrolofCTexamination decreasing attenuation toward the center of a homogeneous
services. object) in the image if uncorrected.
4.4 The extent to which a CTimage reproduces an object or
7.2.1 X-rays produced from electrical radiation generators
a feature within an object is influenced by spatial resolution,
have focal spot sizes ranging from a few millimeters down to
statistical noise, slice plane thickness, and artifacts of the
a few micrometers. Reducing the focal spot size reduces
imaging system. Operating parameters should strike an overall
geometric unsharpness, thereby enhancing detail sensitivity.
balance between image quality, inspection time, and cost.
Smaller focal spots permit higher spatial resolution, but at the
These parameters should be considered for CTsystem configu-
expense of reduced X-ray beam intensity.
rations, components, and procedures. The setting and optimi-
7.2.2 A radioisotope source can have the advantages of
zation of CT system parameters is discussed in Section 9.
small physical size, portability, low power requirements, sim-
4.5 Methods for the measurement of CT system perfor-
plicity, and stability of output. The disadvantages are limited
mance are provided in Section 10 of this practice.
intensity and limited peak energy.
7.2.3 Synchrotron Radiation (SR) sources produce very
5. Significance and Use
intense, naturally collimated, narrow bandwidth, tunable radia-
5.1 This practice is applicable for the systematic assessment
tion. Thus, CT systems using SR sources can employ essen-
of the internal structure of a material or assembly using CT
tially monochromatic radiation. With present technology, how-
technology. This practice may be used for review by system
ever, practical SR energies are restricted to less than
operators, or to prescribe operating procedures for new or
approximately 20 to 30 keV. Since any CT system is limited to
routine test objects.
the inspection of samples with radio-opacities consistent with
5.2 This practice provides the basis for the formation of a
the penetrating power of the X-ray employed, SR systems can
program for quality control and its continuation through
in general image only small (about 1 mm) objects.
calibration, standardization, reference samples, inspection
7.3 The detection system is a transducer that converts the
plans, and procedures.
transmitted radiation containing information about the exami-
nation object into an electronic signal suitable for processing.
6. Basis of Application
The detection system may consist of a single sensing element,
6.1 This practice provides the approach for performing CT
a linear array of sensing elements, or an area array of sensing
examinations. Supplemental information covering specific
elements. The more detectors used, the faster the required scan
7 8
items where agreement between supplier and purchaser are
data can be collected; but there are important tradeoffs to be
necessary is required. Generally the items are application
considered.
specific or performance related, or both. Examples include:
7.3.1 Asingle detector provides the least efficient method of
system configuration, equipment qualification, performance
collecting data but entails minimal complexity, eliminates
measurement, and interpretation of results.
detector cross talk and detector matching, and allows an
arbitrary degree of collimation and shielding to be imple-
7. System Configuration
mented.
7.1 Many different CT examination system configurations
7.3.2 Linear arrays have reasonable scan times at moderate
are possible and it is important to understand the advantages
complexity, acceptable cross talk and detector matching, and a
andlimitationsofeach.Itisimportantthattheoptimumsystem
flexible architecture that typically accommodates good colli-
parameters be selected for each examination requirement,
mation and shielding. Most commercially available CT sys-
through a careful analysis of the benefits and limitations of the
tems employ a linear array of detectors.
available system components and the chosen system configu-
7.3.3 An area detector provides a fast method of collecting
ration.
data but entails the transfer and storage of large amounts of
7.2 Radiation Sources—While the CT examination systems
information, forces tradeoffs between cross talk and detector
may utilize either gamma-ray or X-ray generators, the latter is
efficiency, and creates serious collimation and shielding chal-
used for most applications. For a given focal spot size, X-ray
lenges.
generators (that is, X-ray tubes and linear accelerators) are
7.4 ManipulationSystem—Themanipulationsystemhasthe
several orders of magnitude more intense than isotope sources.
functionofholdingtheobjectunderexaminationandproviding
Most X-ray generators are adjustable in peak energy and
the necessary range of motions to position the examination
intensity and have the added safety feature of discontinued
object between the radiation source and detector. Two types of
radiation production when switched off; however, the poly-
scan motion geometries are most common: translate-rotate
chromaticity of the energy spectrum from an x-ray source
motion and rotate-only motion.
7.4.1 With translate-rotate motion, the object is translated in
7 adirectionperpendiculartothedirectionandintheplaneofthe
As used within this document, the supplier of computed tomographic service
X-ray beam. Full data sets are obtained by rotating the
refers to the entity that physically provides the computed tomographic services.The
supplier may be a part of the same organization as the purchaser, or an outside
examination object between translations by the fan angle of the
organization.
beam and again translating the object until a minimum of 180
As used within this document, the purchaser of computed tomographic services
degrees of data have been acquired. The advantage of this
refer to the entity that requires the computed tomographic services. The purchaser
may be a part of the same organization as the supplier, or an outside organization. design is simplicity, good view-to-view detector matching,
E1570–00
flexibility in the choice of scan parameters, and ability to or rejecting the examination object, subject to the operator’s
accommodate a wide range of different object sizes including interpretation of the CT data.
objects too big to be subtended by the X-ray fan. The
7.7.1 Generally, CT image display requires a special graph-
disadvantage is longer scan time.
ics monitor; television image presentation is of lower quality
7.4.2 With rotate-only motion, a complete view is collected but may be acceptable. Most industrial systems utilize color
by the detector array during each sampling interval. A rotate- displays. These units can be switched between color and
only scan has lower motion penalty than a translate-rotate scan gray-scale presentation to suit the preference of the viewer, but
and is attractive for industrial applications where the part to be it should be noted that gray-scale images presented on a color
examined fits within the fan beam and scan speed is important. monitor are not as sharp as those on a gray-scale monitor. The
use of color permits the viewer to distinguish a greater range of
7.5 Computer System—CT requires substantial computa-
variations in an image than gray-scale does. Depending on the
tional resources, such as a large capacity for image storage and
application, this may be an advantage or a disadvantage.
archival and the ability to efficiently perform numerous math-
Sharply contrasting colors may introduce false, distinct defini-
ematical computations, especially for the back-projection op-
tion between boundaries. While at times advantageous, un-
eration. Computational speed can be augmented by either
wanted instances can be corrected through the choice of color
generalized array processors or specialized back-projection
(or monochrome) scale.
hardware. The particular implementations will change as
computerhardwareevolves,buthighcomputationalpowerwill 7.8 Data Storage Medium—Many CT examination applica-
remain a fundamental requirement for efficient CT examina- tions require an archival-quality record of the CT examination.
tion. A separate workstation for image analysis and display This could be in the form of raw data or reconstructed data.
often is appropriate. Therefore, formats and headers of digital data need to be
specified so information can be retrieved at a later date. Each
7.6 Image Reconstruction Software— The aim of CT is to
archiving system has its own specifics as to image quality,
obtain information regarding the nature of material occupying
archival storage properties, equipment, and media cost. Com-
exact positions inside an examination object. In current CT
puter systems are designed to interface to a wide variety of
scanners, this information is obtained by “reconstructing”
peripherals.As technology advances or needs change, or both,
individual cross-sections of the examination object from the
equipment can be easily and affordably upgraded. The exami-
measured intensity of X-ray beams transmitted through that
nation record archiving system should be chosen on the basis
cross section. An exact mathematical theory of image recon-
of these and other pertinent parameters, as agreed upon by the
struction exists for idealized data. This theory is applied
supplier and purchaser of CT examination services. The
although the physical measurements do not fully meet the
reproductionqualityofthearchivalmethodshouldbesufficient
requirements of the theory. When applied to actual measure-
to demonstrate the same image quality as was used to qualify
ments, algorithms based on this theory produce images with
the CT examination system.
blurring and noise, the extent of which depends on the quantity
and quality of the measurements. 7.9 Operator Interface—The operator interface determines
much of the function of the rest of the CT
...
Questions, Comments and Discussion
Ask us and Technical Secretary will try to provide an answer. You can facilitate discussion about the standard in here.